I've been using paperpile in conjunction with Google Docs for over a year now, and have used it in writing a number of manuscripts with international collaborators. Overall, it's been a great experience. Not having to email back and forth bibtex or endnote databases and praying they work on someone else's setup is tremendously liberating! The Chrome browser integration is handy, letting you directly import citations from Google Scholar or Pubmed and automatically grab pdfs without ever going to the journal webpage - great for collecting articles for literature reviews, or adding to "read someday" lists while you're searching for something else. The bibliography formatting seems (to me, anyway) more straight-forward than with e.g. endnote, and I've found myself having to do less hand editing of reference sections as a result. The few times I've contacted technical support, the developers have been very responsive and helpful. Can't recommend it highly enough - it's the first reference manager I've actually enjoyed (rather than dreaded) using.